Azure Stack provides a hybrid Azure platform that does is separate from the Azure Public Cloud. Currently we use the cloudName to generate the Azure environment (https://github.com/vmware-tanzu/velero-plugin-for-microsoft-azure/blob/22b6776fba44b7f521afce56de6e20f1b7373855/velero-plugin-for-microsoft-azure/common.go#L50). Azure Stack appears to need a different path to generate the environment (https://raw.githubusercontent.com/Azure/go-autorest/master/autorest/azure/environments.go). There may be other issues as well.